1.) Technical Field
The invention relates to chemical vapor deposition (CVD) of metal onto semiconductor structures, and in particular to metal CVD processes employing a metal precursor gas which tends to attack underlying metal layers.
2.) Background Art
Certain types of microelectronic semiconductor structures have plural interconnected conductor layers consisting of conductive materials such as polysilicon, silicon, aluminum, tungsten and so forth. The conductor layers are separated by insulating layers consisting of an insulating material such as silicon dioxide. An interconnection between a pair of conductor layers is provided prior to the formation of the overlying conductor layer by etching a contact opening 10 (as shown in FIG. 1) through the insulator layer 15 overlying the lower conductor layer 20 on a semiconductor wafer 22. The underlying conductor layer 20 is typically silicon or polysilicon. (Alternatively, the underlying conductor layer 20 is the wafer 22 and is crystalline silicon.) Typically, the silicon dioxide insulator layer 15 is about 2000 Angstroms thick while the contact opening 10 is about 500 Angstroms wide, so that the contact opening 15 tends to be deep and narrow, and therefore somewhat difficult to completely fill with a metal contact layer. Filling the contact opening 15 is preferably accomplished by chemical vapor deposition (CVD) of a tungsten contact layer 25 because CVD of tungsten tends to completely fill the deep narrow contact opening 15 better than many other metals and processes. However, tungsten does not adhere well to the silicon dioxide insulating layer 15 and, moreover, forms a relatively high-resistance Schottky barrier contact with the underlying silicon conductive layer 20.
The problems of high contact resistance and poor adhesion is addressed by physical vapor deposition (PVD) of an underlying adhesion layer 30 of titanium prior to the CVD of the tungsten contact layer 25. Preferably, the adhesion layer 30 is between about 100 and 200 Angstroms thick. The titanium adhesion layer 30 adheres well to the underlying silicon conductor layer 20 and provides a lower contact resistance to the silicon conductor layer 20. However, this approach creates another problem in that the tungsten CVD process employs a tungsten precursor gas such as tungsten hexafluoride (WF.sub.6) which reacts with the titanium adhesion layer 30 so as to form an alloy of titanium, tungsten and fluorine. In order to shield the titanium adhesion layer 30 from such attack during the tungsten CVD process, a protective titanium nitride layer 35 is formed by PVD over the titanium layer 30 prior to the CVD formation of the tungsten layer 25. Preferably, the titanium nitride protective layer 35 is between about 500 and 1000 Angstroms thick and is relatively immune from attack by the WF.sub.6 gas introduced during the subsequent tungsten CVD process. The titanium nitride layer 35 forms an ohmic contact with the underlying titanium layer 30 and with the overlying tungsten contact layer 25.
However, the titanium nitride protective layer 35 does not cover all of the titanium layer 30. The problem is that during the PVD deposition of, first, the titanium layer 30 and, second, the protective titanium nitride layer 35, a circular clamp ring 40 shown in FIG. 2 covers the periphery of the wafer 22 near the wafer edge 24 to hold the wafer 22 in place and to provide better thermal contact between the wafer 22 and the wafer stage inside a PVD reactor. Titanium tends to penetrate under the clamp ring 40 while titanium nitride penetrates thereunder to a far lesser extent. As a result, the titanium adhesion layer 30 underlies the clamp ring 40 by a significant distance (about 0.1 millimeters) while the titanium nitride protective layer 35 essentially stops at the edge of the clamp ring 40, as shown in FIG. 2. This leaves a peripheral annulus 30a of the titanium adhesion layer 30 unprotected by the titanium nitride layer 35, the unprotected annulus 30a being about 0.1 millimeters wide.
Where physical vapor deposition steps of, first, titanium and, second, titanium nitride are performed sequentially in different reactor chambers of a multiple reactor processing system, the unprotected or exposed portion of the titanium layer 30 tends to be even larger. The problem is caused by misalignment of the wafer 22, due to positioning inaccuracies of the wafer-handling apparatus of the PVD system. Specifically, the wafer-handling apparatus of a multiple reactor system delivers the wafer into a reactor chamber using pre-calibrated positioning information. The position calibration is not always adequate to place the same wafer in the same relative position in each of the different chambers in successive processing steps, giving rise to exposure of a greater portion of the titanium layer 30 due to misalignment of the protective titanium nitride layer 35 relative thereto. In such a case, the exposed portion 30a is in the shape of a crescent, rather than an annulus, whose maximum width can be on the order to 1 to 2 mm.
Subsequently, during the CVD formation of the tungsten contact layer 25, the unprotected titanium peripheral annulus (or crescent) 30a is attacked by the WF.sub.6 gas introduced during the tungsten CVD process. This forms an alloy of titanium, tungsten and fluorine, TiW.sub.x F.sub.y, where x and y can be determined by chemical analysis. Near the edge 30b of the titanium annular periphery 30a, this alloy tends to penetrate to the underlying silicon dioxide layer 15, which causes the titanium layer 30 to delaminate beginning at the edge 30b. During subsequent processing steps, this delamination creates TiW.sub.x F.sub.y particle contamination, greatly reducing process yield. There has seemed to be no way of avoiding such contamination.